Modern medicine uses many medical procedures where electrical signals or currents are received from or delivered to a patient's body. The interface between medical equipment used in these procedures and the skin of the patient is usually some sort of biomedical electrode. Such an electrode typically includes a conductor which must be connected electrically to the equipment, and a conductive medium adhered to or otherwise contacting skin of a patient.
Among the therapeutic procedures using biomedical electrodes are transcutaneous electronic nerve stimulation (TENS) devices used for pain management; neuromuscular stimulation (NMS) used for treating conditions such as scoliosis; defibrillation electrodes to dispense electrical energy to a chest cavity of a mammalian patient to defibrillate heart beats of the patient; and dispersive electrodes to receive electrical energy dispensed into an incision made during electrosurgery.
Among diagnostic procedures using biomedical electrodes are monitors of electrical output from body functions, such as electrocardiogram (ECG) for monitoring heart activity and for diagnosing heart abnormalities.
For each diagnostic, therapeutic, or electrosurgical procedure, at least one biomedical electrode having an ionically conductive medium containing an electrolyte is adhered to or otherwise contacting mammalian skin at a location of interest and also electrically connected to electrical diagnostic, therapeutic, or electrosurgical equipment. A critical component of the biomedical electrode is the conductive medium serving as the interface between mammalian skin and diagnostic, therapeutic, or electrosurgical equipment.
Biomedical electrodes are used among other purposes to monitor and diagnose a patient's cardiovascular activity. Diagnostic electrodes are used to monitor the patient immediately and are only applied to the patient for about five to ten minutes. Monitoring electrodes are used on patients in intensive care for up to three days continuously. Holter electrodes are used to monitor a patient during strenuous and daily activities.
All of these biomedical electrodes are used to record cardiovascular activity although each electrode requires specific features to be successful. The diagnostic electrode does not have to remain adhered to a patient for extensive periods but does have to adhere to hairy, oily, dry and wet skin effectvely for the five to ten minutes of use. The monitoring electrode has to adhere for a longer period of time although the patient is often immobile during the monitoring period. The Holter electrode is susceptible to disruption from adhesion due to physical motion, perspiration, water, etc., and therefore requires the best adhesion and at the same time comfort and electrical performance.
The ionically conductive medium which serves as an interface between the skin of a mammalian patient and the electrical instrumentation ranges from conductive gels and creams to conductive pressure sensitive adhesives. However, while conductive media can be pressure sensitive adhesives, for monitoring or Holter biomedical electrode use, such conductive adhesives are not adequate alone to maintain adhesion to mammalian skin. Hypoallergenic, hydrophobic pressure sensitive adhesives are employed around the conductive medium to provide the required mammalian skin adhesion. U.S. Pat. No. 5,012,810 (Strand et al.) and U.S. Pat. Nos. 4,527,087; 4,539,996; 4,554,924; and 4,848,353 (all Engel) disclose biomedical electrodes which have a hydrophobic pressure sensitive adhesive surrounding the conductive medium.
Conductive media are typically hydrophilic and need water or an aqueous ionic system to provide required ionic conductivity between mammalian skin and electrical diagnostic, therapeutic, or electrosurgical instrumentation. Hydrophilic pressure sensitive adhesives generally have less skin adhesion than hydrophobic adhesives. Sometimes, such hydrophilic pressure-sensitive adhesives can not function as adhesives to mammalian skin for the period of time necessary to complete a medical procedure.
To improve the tack of conductive hydrogels, tackifiers have been added. U.S. Pat. No. 4,593,053 (Jevne et al.) discloses the addition of poly-2-acrylamido-2-methyl propane sulfonic acid, its salts, polyacrylic acid, polystyrene sulfonic acid or salts thereof, karaya, xanthan, guar, or locust bean gums in an amount of 2 to 20 weight percent to increase tackiness of gel containing polyvinylpyrrolidone and polyvinyl alcohol.
Also, adhesive additives have been used in the formation of the solid phase of a matrix useful as a medical bandage. U.S. Pat. No. 4,307,717 (Hymes et al.) discloses the addition of vinyl acetate dioctyl maleate copolymer to intensify the tackiness of the bandage.
Rather than employ a single phase hydrogel for the conductive medium, U.S. Pat. No. 4,588,762 (Mruk et al.) discloses a heterogeneous, pressure-sensitive, electrically conductive adhesive for disposable biomedical electrodes consisting of a viscoelastic polymeric adhesive phase and an electrically conductive aqueous phase containing a water receptive polymer, humectant, and an electrolyte. Both phases are intimately interdispersed and the adhesive is applied as a relatively thin film on a supporting substrate. The final film constitutes a heterogeneous system in which the aqueous zones or islands defined by the water receptive polymer extend through the thickness of the film and are distributed throughout the expanse of a continuous matrix consisting of the adhesive polymer. However, to be electrically conductive, such zones or islands must contact both mammalian skin and electrically conductive materials in the biomedical electrode. If such zones or islands do not transverse the thickness of the film, they are electrical dead ends.
PCT International Publication WO 91/09633 (Asmus) discloses a two-phase composite of a continuous phase of a pressure sensitive adhesive matrix having dispersed therein swollen, discrete gel particles. If such particles were ionically conductive, such particles could not transmit electrical signals between mammalian skin and electrical instrumentation unless such particles transversed the thickness of the continuous phase.